1. Field of the Invention
The present invention relates to fluid control systems. More particularly, the present invention relates to a multiple solenoid valve, closed loop, hydraulic control system suitable for implementation in aeronautical material dispersal applications.
2. Description of the Related Art
The inventor hereof is also an inventor of U.S. Pat. No. 5,279,481 to Trotter et al. (the “'481 patent”), for Airborne Liquid Spreading System, the entire disclosure of which is hereby incorporated by reference thereto. The '481 patent is directed to an airborne system for dispersing a liquid or other flowable material from an aircraft to the ground below the aircraft. Specific coverage rates are controlled and maintained utilizing a control system, which includes a digital controller that repeatedly calculates a differential pressure across the doors of container holding the liquid or material in the aircraft, and then operates to vary the position of the doors to maintain a constant material flow rate as the differential pressure varies. The control system then closes the doors when a specified volume of material has been dispersed. The control system in the '481 patent utilizes a hydraulic drive system where a digitally controlled proportional hydraulic valve couples hydraulic fluid to drive a rotary actuator to control the position of the doors. A position sensor is coupled to the rotary actuator to provide feedback to the digital controller such that the proportional valve can be effectively controlled within the closed-loop system.
The use of hydraulic proportional control valves implies certain issues in the system design, operation and maintenance. Proportional valves are more expensive, more complex, heavier, and less reliable than simple open-close valves that are typically used in simple open and closed flow control applications. However, simple open-close valves are not well suited to precise control applications because of the “bang-bang” open and close actuation cycle they exhibit. The abrupt operation of such valves induces stresses in mechanical parts, jars loads, and makes precise and intricate control movements difficult. Thus, although simple and economical, open and closed valves are not presently well suited for use in closed loop control system designed for precise and intricate transfers of fluid and control of mechanical processes in a closed loop control system.
By way of an example, a hydraulic proportional valve and hydraulic rotary actuator can be readily employed in an application where it is useful to quickly rotate the actuator to a known position then maintain position within defined error tolerances, particularly in the case of a closed loop positioning application. If open and closed hydraulic valves were substituted and specified with sufficient flow rate capabilities to control the actuator for maximum angular velocity of movement, then the hold-position tolerance required during steady state positioning could not be well maintained due to the high flow rates and actuation time delay inherent in actuating larger “bang-bang” valves. Additionally, acceleration and deceleration of the actuator are not easily or even readily controllable to the extent desirable in precisely operating control system. Thus is can be appreciated that there is a need in the art for a system and method of utilizing open and closed fluid valves in a closed loop control system that address the problems in the art of precise fluid transfer and precise control system operations.